A Schottky-barrier CCD on GaAs

Because of it´s high electron-mobility GaAs is a promising semiconductor for high speed CCDs. To avoid the problems encountered with MIS-structures (e.g., high surface state density, frequency dependent C(U)-characteristics) a CCD with Schottky-contacts was proposed (Schuermeyer 1972). To our knowledge no operating GaAs-CCD has been reported until now. Basically the structure of our CCD is that of a GaAs MESFET with a sequence of gates. During operation, electrons are injected from the source into the depleted n-type layer. By proper biasing of the Schottky-gates the injected charge is stored or transferred along the interface between active layer and substrate. The signal is detected by use of a monolithically integrated read-out MESFET-amplifier. A three phase-four stage CCD was fabricated on sulphur-implanted active layers on semi-insulating GaAs. The ohmic contacts consist of a layer sequence of GeAuCrAu. CrAu-Schottky-contacts with 6.5 µm in length and 200 µm in width were used. Interelectrode spacing was 1.5 µm. SiO2was sputter-deposited for electrode crossover and passivation. Operation of the CCD at 500 kHz showed correct single pulse and step response. The number of stages were too small for accurately measuring the transfer inefficiency, which is estimated to be smaller than 5.10^-3. Elements with 8 or 16 stages for determining the transfer inefficiency and the maximum frequency of operation are presently under investigation.